This invention relates to electroplating metal layers onto substrates and electropolishing metal layers on substrates. More specifically, it relates to apparatus for controlling the composition, flow, and potential distribution of electrolyte while electroplating or electropolishing a silicon wafer.
Damascene processing is a method for forming metal lines on integrated circuits. It is a significant departure from traditional methods that require blanket deposition and subsequent patterning of aluminum. In comparison to such traditional processes, Damascene processing requires fewer processing steps and offers a higher yield. In Damascene processes, copper is a favored over aluminum because of its higher conductivity and resistance to electro migration.
In a typical Damascene process, copper is deposited in at least two steps. First, the process deposits a very thin layer of the metal by physical vapor deposition (PVD). Then, the process forms a thicker copper electrofill layer by electroplating. The PVD process is typically sputtering. One example of a commercially successful apparatus that electroplates copper onto wafer active surface is the SABRE™ electroplating apparatus available from Novellus Systems, Inc. of San Jose, Calif. and described in U.S. patent application Ser. No. 08/969,984, “CLAMSHELL APPARATUS FOR ELECTROCHEMICALLY TREATING SEMICONDUCTOR WAFERS” naming E. Patton et al. as inventors, and U.S. application Ser. No. 08/970,120, both filed Nov. 13, 1997, which are herein incorporated by reference in their entirety and for all purposes.
Electroplated copper should fill Damascene trenches from the “bottom-up.” If instead the copper plates on the top and side-walls of the Damascene trenches, voids form in the conductive lines, reducing conductivity and causing the integrated circuit to be unusable. The plating electrolyte (often referred to as the “plating bath”) composition helps control the conformation of electroplated copper. Certain organic additives known as “accelerators” or “brighteners” significantly improve the copper feature filling when added to the electrolyte. In fact, a significant technical challenge in plating copper on integrated circuits involves maintaining stable additives in the electrolyte. If the additives degrade with use, one cannot achieve consistent bottom-up plating. A discussion of bath degradation and maintenance strategies can be found in “Use of On-Line Chemical Analysis for Copper Electrodeposition,” R. J. Contolini, J. D. Reid, S. T. Mayer, E. K. Broadbent, and R. L. Jackson, Advanced Metallization Conference, 1999, Sep. 28-30, 1999, Orlando, Fla., Paper # 27. In general, a controlled composition of a plating bath is essential to maintain good bottom-up electroplating, uniformity and other desirable plating characteristics. While certain organic additives promote bottom-up plating, other compounds interfere with such plating. Some of the interfering compounds are decomposition products of the desirable accelerators. It has been found, for example, that poor plating is often associated with decomposition of accelerators. Many plating baths contain accelerators such as dimercaptopropane sulfonic acid (SPS) or N-dimethyldithiocarbamic acid (DPS). These can breakdown to their monomers (e.g., mercaptopropane sulfonic acid (MPS)). Small amounts of MPS in an SPS or DPS bath can substantially degrade bottom-up filling.
Additive degradation may be mitigated by periodically dumping an old bath and adding a fresh bath (sometimes termed a “bleed and feed” or a “bath replenishment” procedure). U.S. Pat. No. 5,352,350 issued to Andricos et. al. describes an embodiment of this approach. In theory, this approach maintains the concentration of “poisons” (e.g., MPS or other breakdown product) at an acceptable steady state value. Unfortunately, it produces a substantial volume of waste and requires a continuous detailed bath analysis using maintenance metrology. Waste generation is environmentally problematic and requires costly treatment. Further, as wafer diameters increase (as they will continue to do), the amount of required dumping increases.
In another approach, the plating apparatus may include an adsorption column (e.g., an activated carbon fluidized bed) to remove poisons. Generally, such beds lack the specificity to remove only the unwanted poisons. Thus, this approach typically strips all additives from the plating bath to create a “virgin” solution. This solution is then reintroduced to the main bath together with appropriate levels of fresh additive, Unfortunately, this approach is often uneconomical because it requires (i) processing of large volumes of plating bath, (ii) large quantities of fresh accelerators and other expensive additives, and (iii) a large carbon filter, which need to be replaced frequently.
Most organic additive breakdown processes occur at the anode surface. To reduce breakdown, plating systems may employ copper anodes containing 0.02 to 0.04% phosphorus. Such anodes form a surface film with better tenacity and less particle generation than non-phosphourus containing anodes and also act as a protective diffusion barrier for brighteners (see Modern Electroplating, Frederick A. Lowenheim, editor, Third edition, pg 192). Still, the film has a particulate morphology and accumulates breakdown products. Also, it has been found that the bath plating quality (as evidenced by copper layer conformation and defects) is strongly sensitive to disturbances in the anode film caused by stirring and other mechanical perturbations commonly employed in modern electroplating apparatus.
There are generally two classes of anodes that are used in metal plating: consumable (also referred to as active) anodes, and non-consumable (also referred to as “dimensionally stable” and non-reactive) anodes. The reactions of the active anode for plating copper are simple and balanced (no overall depletion or generation of new species). Copper ion in the solution are reduced at the cathode and removed from the electrolyte, simultaneously as copper is oxidized at the anode and copper ions added to the electrolyte. In contrast, the reactions in a non-consumable system are unbalanced. The two reactions are:H2O→½O22H++2e−  (anode) Cu+2+2e−→Cu  (cathode). 
U.S. Pat. No. 4,469,564 issued to Okinaka et al. describes a copper electroplating system in which the non-consumable anode is surrounded by a cation exchange membrane. The membrane prevents passage of organic additives and anions, and thereby prevents the organic additives from contacting the non-consumable anode, while allowing passage of positive ions (generally hydrogen) to pass cationic current. When the membrane is present, the anode chamber will accumulate hydrogen ion. Accordingly, “the feature is especially advantageous for copper electroplating processes using non-consumable electrodes because of the high consumption of additives and that the copper can be added to the cathode side of the membrane so that acid copper ions need not pass through the membrane.” Unfortunately, the resistance to ion mass transport across such membranes is great.
U.S. Pat. No. 5,162,079 issued to Brown describes an electroplating system in which the non-consumable anode is enclosed in a compartment having a nonporous anion or cation exchange membrane with a means of flushing the anode compartment to maintain the acid concentration there.
What is needed therefore is improved electroplating technology that reduces the rate at which additives break down, minimizes power consumption, improves the bath stability/longevity, and minimizes chemical waste generation.